The present invention relates to satellite antenna systems. In particular, the present invention relates to a deployable phased array of reflector antennas that provides scanning capability using reflector antennas as elements.
Spaceborne communication applications often rely on deployable reflector antennas to achieve high gain. The deployable reflector antenna uses one or more feeds located at or near the reflector focal point, for example, to receive energy focused by the reflector at the focal point. An alternative type of antenna, the direct radiating phased array antenna, is built using a large number of direct radiating elements spaced closely together on a lattice, and is often impractical for space applications.
In many communication applications, an antenna with a large effective aperture size is desired. The aperture size refers to the physical size of the antenna, and, as the aperture size increases, the sensitivity or "gain" of the antenna increases, with a concomitant reduction in beamwidth. A large aperture size thus produces a narrow beam that allows an antenna to receive or transmit energy from or to a very precise point. For example, a large aperture antenna is more effective at collecting, focusing, finding and pinpointing the energy emitted by a distant star.
In addition to having a large aperture, many antennas preferably have agile scan capability, which is the ability to rapidly (i.e., electronically, instead of mechanically) scan a transmit or receive beam over a wide angular range. In a phased array antenna, a set of amplitude and phase control electronics drive each radiating element. The control electronics are typically quite flexible and allow a phased array antenna to achieve an enormous angular range. For example, a phased array antenna may have an angular range of .+-.30 to .+-.45 degrees. Unfortunately, as the aperture size of a phased array antenna increases, the amount of radiating elements and associated control electronics drastically increases, with a concomitant increase in power consumption, thermal dissipation and weight. The complexity of the structural design and the deployment also increase drastically. In other words, large aperture phased array antennas are impractical from economic and engineering standpoints.
A deployable mesh reflector antenna, on the other hand, readily achieves very large aperture sizes with very low weight and stow volume. As a reflector antenna increases in size, however, its angular steering range becomes more limited due to optical aberrations which degrade antenna sensitivity (or "gain"). Although longer focal lengths or multiple feeds may be used in a reflector to increase the angular scanning range, the fact remains that the angular scan range of a reflector decreases as the reflector size increases. Furthermore, as the aperture size increases and the beam width narrows (which in most instances is a desirable condition that creates a high power beam), an increasingly smaller feed handles an increasing amount of power. However, the amount of power that practical feeds and electronics can handle is limited by breakdown, multi-paction or heating.
Therefore, in the past, practical reflector antennas have been limited to approximately 10 to 20 beamwidths of scan, and signal power levels are constrained. A phased array antenna, on the other hand, has the ability to scan several hundred beamwidths. Further, the phased array distributes energy over numerous antenna elements and has the capability for handling much higher levels of power. As noted above, however, it is usually impractical to construct a large aperture phased array antenna.
Spaceborne antennas, of course, reach orbit in a launch vehicle. Launch vehicles are extremely expensive, and any reduction in size and weight generally results in a reduced cost to launch. Thus, although large aperture antennas are desirable, the aperture size has, in the past, been limited by the launch cost, size of the launch vehicle, and the extent to which the antenna can be folded or packed together into the launch vehicle. Thus, there is a further need for a cost effective, light weight, compact large aperture antenna that is economical to launch.
A need has long existed in the industry for a new antenna that overcomes the problems noted above and previously experienced.